Most conventional image sensors operate by sensing an image projected onto an array of discrete image sensor elements. The electrical response of each image sensor element is proportional to the total light falling within its boundaries. The electrical pattern held by the array of discrete image sensor elements is retrieved by sequentially interrogating the electrical response of each of the sensor elements. The resulting sensor output is converted into digital representations (e.g., numbers ranging from 0-255, or any other convenient range) of the impinging light intensity. These numbers can be used to directly construct a video or print image (with the relative intensity of the sensor output of each image sensor element being reproduced as a single picture element or "pixel"), or alternatively can be modified using conventional image processing algorithms to provide edge enhancement, half-tone descreening, image compression, noise reduction, or other known image enhancement techniques. For example, one commonly employed image processing technique requires that those numbers corresponding to adjacent or nearby sensor elements in the array of image sensor elements be combined in a weighted average. This technique is equivalent to convolution with a kernel of weight coefficients. Often these weighting factors are Gaussian functions of distance from the center of the kernel. Many image processing functions such as edge detection, Gaussian pyramid compression, and edge enhancement have been implemented using Gaussian kernels.
Unfortunately, Gaussian image processing kernels often extend over a large number of neighboring pixels. Consequently, if the kernel possesses N elements and the image consists of M pixels, evaluation of just one convolution of the image with a Gaussian kernel nominally requires NXM multiplications and additions (note that symmetries in the Gaussian kernel may reduce somewhat the required number of calculations). Given a common scanning resolution of 300 dots per inch, a typical number of pixels for a page sized image is on the order of M=10 million pixels. Even a single 5.times.5 average of neighboring pixels would require about 250 million multiplications and additions. Such a computational task can require several seconds or longer per convolution, even on high performance massively parallel processors. The necessity for speed is particularly critical in real-time applications, applications requiring a varying kernel size, or applications requiring user feedback.
One possible technique for high speed convolution relies on varying sensor size, since a measured electrical output of a sensor array is inherently a convolution between image intensity and spatial dependence of sensor response. If the spatial response (sensor size) of the detector is adjusted to match a desired kernel, the requisite convolutions with that kernel are implicitly computed with a speed comparable to the sensor array readout time. In effect, by adjusting the size of the sensor, one can perform a desired convolution at the same speed as the image is stored, without requiring any additional computer processing time.
However, conventional sensor fabrication techniques do not allow for externally controlled, real time variations in the size of light sensitive areas of individual sensor elements. In conventional sensor arrays (usually composed of Schottky, p-n, or p-i-n sensor elements), the light sensitive areas of the individual sensor elements are designed to be discrete, fixed size and non-overlapping with neighboring sensor elements. The light falling within one pixel induces an electrical response in only that corresponding element. Typically, the radiation sensitive zone or area is defined by physically patterning the active region of each sensor element. Accordingly, the shape, size, or position of the light sensitive active region of a sensor cannot be altered after fabrication. The fixed binary response of patterned sensors is thus inappropriate for many image processing convolution tasks.
Accordingly, the present invention relates to radiation sensor elements with radiation responsive areas that can be spatially altered by external control. Since the size of the responsive area can be altered, response to incident radiation is not limited to a predetermined area, but can be varied as desired. It is even possible that the responsive area of a radiation sensor element can be extended to overlap the responsive area of a neighboring element. The invention also is capable of permitting implementation on the radiation sensor of convolutions with certain classes of useful image kernels. In a preferred embodiment, a radiation sensor in accordance with the present invention includes a semiconductor device having a collection electrode and a radiation sensor positioned in electrical contact with the collection electrode. The radiation sensor is configured to produce a detectable response at the collection electrode upon incidence of radiation (e.g., X-rays, ultraviolet, visible, infrared, or other detectable radiation) in a responsive zone. A gate electrode layer is separated from the radiation sensor by a dielectric layer, with adjustments to voltage applied to the gate electrode inducing changes in areal extent of the responsive zone of the radiation sensor.
In a most preferred embodiment, the radiation sensor is a p-i-n or n-i-p type sensor having a collection electrode for collecting charge generated by radiation incident on the i-layer (typically intrinsic amorphous silicon). To simplify retrieval of data, a pass transistor can be connected to receive charge from the collection electrode for user controlled passing of charge to a data line. Preferably, the pass transistor has a drain connected to the collection electrode, a source connected to the data line and separated from the drain, and a pass transistor gate electrode that can be activated to promote passage of charge from the drain to the source, and ultimately to a data line for measurement.
In another alternative embodiment of the present invention having particularly high sensitivity, the collection electrode is configured to act as a pass transistor gate electrode. The pass transistor gate electrode is layered in electrical contact with radiation sensor. Depending upon the amount of received photogenerated charge by the radiation sensor, the pass transistor gate electrode controls the amount of current passing between a drain and a source layered above the pass transistor gate electrode. This change in current flow between the source and the drain is provided to a data line as a measure of the magnitude of the photogenerated charge.
Other objects and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the drawings and preferred embodiments.